Degrees of Quantumness: Shades of Gray in Particle-Wave Duality

Degrees of Quantumness: Shades of Gray in Particle-Wave Duality

Article excerpt

Is light made of particles or waves? The answer, according to quantum physics, is both. Depending on the situation, particles of light--and particles of matter too--sometimes contradict themselves and act like waves. But between these two extremes, there's a range of behaviors. Scientists have now demonstrated those intermediates in a conspicuous way.

The new research is a variation on the so-called double-slit experiment, a staple of introductory quantum theory courses. In the classic version, light passes through two slits in an opaque screen and hits another screen some distance away. Crests and troughs of light waves emerging from each slit add together or cancel each other out, depending on howthey overlap, and create an interference pattern of light and dark stripes on the screen. This phenomenon has been demonstrated not only with photons but also with electrons, and even whole atoms.

From the quantum perspective, however, light is a stream of photons. To explain the interference pattern, physicists say that each photon travels through both slits simultaneously and then interferes with itself on the other side.

The additional twist is that, according to quantum theory, the interference pattern--a wave phenomenon--would disappear if one knew for sure through which slit each photon went. In principle, detectors at the slits would register a photon's passage without capturing the particle. In that situation, the photon would have chosen one slit or the other, thereby behaving like an old-fashioned, classical-physics particle.

Physicists suspected that it's possible to extract only partial information about a particle's route. …